Conventional antenna circuits including antenna gain correction circuits are available as described in, for example, Japanese Laid-Open Patent Publication No. 62-260403. The following will describe a conventional antenna circuit with reference to the accompanying drawings.
FIG. 4 shows an example in which an antenna circuit is used for a small television receiver meeting US specifications. Basically an antenna gain correction circuit 42 comprises a parallel circuit of an inductor 44, a capacitor 47, and capacitors 46 and 48. In this circuit, the capacitors include a variable-capacitance diode 46 which varies in capacitance in response to the control of a receiving circuit 43 and the capacitors 47 and 48 which have fixed capacitances. Reference numeral 45 denotes a choke coil.
The inductor 44 is a chip inductor having a nominal inductance of 0.33 μH and a self-resonant frequency of 350 MHz. In response to the control of the receiving circuit 43, the variable-capacitance diode 46 varies in capacitance from 2 pF to 12 pF and the combined capacitance of the capacitors changes from 4.5 pF to 9 pF. As a result, the impedance characteristics of the antenna gain correction circuit 42 change and the self-resonant frequency changes between 85 MHz and 120 MHz. For example, when Channel 2 is received, the antenna gain correction circuit 42 has a self-resonant frequency of 85 MHz and an impedance of about 300Ω.
As the low band of the VHF band is sequentially selected from Channel 2 to Channel 6, the capacitance of the variable-capacitance diode 46 is changed in response to the control of the receiving circuit 43 and the self-resonant frequency of the antenna gain correction circuit 42 is successively changed from 85 MHz to 120 MHz. As a result, the impedance of a receiving channel is also successively changed from about 300Ω to 100Ω and the impedance characteristics of the antenna gain correction circuit 42 are optimized for each channel.
As described above, in the conventional antenna circuit, the antenna circuit is optimized by changing the impedance characteristics of the antenna gain correction circuit 42. However, the use of the variable-capacitance diode complicates the structure and increases the cost.